Bug report

phpstan thinks is_subclass_of always returns false if the first argument is a string. If phpstan can't resolve the strings to specific classes in the code base, and so can't determine itself whether is_subclass_of is true or false, it should assume the output is bool, not false.

This clearly has overlaps with (and may even duplicate one or more of) issues #6305, #5369, #3979 and #2755, but my case was dealing with the first argument being a string specifically.

Code snippet that reproduces the problem

https://phpstan.org/r/e4ead7a9-f323-4dbe-af78-ac8a69e518eb

$result = \is_subclass_of( '\\My\\Namespace\\MyClass', '\\My\\Namespace\\MyBaseClass', true);

Expected output

No error reported by phpstan. \My\Namespace\MyClass and \My\Namespace\MyBaseClass are unknown to it, and this call to is_subclass_of is valid.

Did PHPStan help you today? Did it make you happy in any way?

Yes. It has helped me squash several potential bugs already today.